The present invention relates generally to display apparatus of the type employing cathode ray tubes (CRT's). More particularly, the invention relates to pincushion distortion correction in display systems using separate major and minor deflection signals.
CRT display systems are well known for presenting a wide range of information to a viewer. In its simplest form, a CRT comprises an electron gun, a surface which emits visible light when struck by electrons and a system of coils and or plates for focusing and directing the electrons onto the sensitive surface. When presenting a pattern on a relatively flat-faced CRT using magnetic deflection, the phenomenon of pincushion distortion is commonly encountered. This geometric phenomenon has resulted in commercially available pincushion correction modules which accept linear deflection signals, predistort them prior to final amplification, and produce a linear pattern on the CRT face.
Commercially available pincushion correction modules are attractive for several reasons, such as relatively small size and ease of use. Primarily, however, these modules represent a solution to a design problem which need not drastically alter the remainder of the display system. These modules are usually adjustable for the geometry of the system in which they are used, so for practical purposes they represent a "black box" solution to pincushion distortion which is transparent to the remainder of the display system.
In systems wherein some or all of the information to be displayed consists of letters, numbers and other symbols the electronics needed to generate the deflection signals are sometimes divided into two separate systems. One system, the major deflection system, comprises low frequency, high power circuits to generate the signals which grossly position the electron beam on the CRT face. The second system, the symbol or minor deflection system, comprises high frequency, low power circuits to generate the signals which direct the electron beam to "draw" a particular character. The use of two separate systems is motivated, in part, by the desire to avoid the use of circuit components which are capable of performing at high power and high bandwidth.
Correction of pincushion distortion in such dual deflection systems is possible by several methods. The primary requirement is that the correction applied to the minor deflection signal must be a function of the gross position of the symbol on the CRT face. Commercial pincushion correction modules may be used in single deflection systems. This is analogous to combining the major and minor deflection signals of a dual deflection system into a single deflection signal, then inputting the combined signal to a correction module and applying the resulting signal to a single system of amplifiers and coils. This method discards many of the advantages of a dual deflection system, however. Another method is described in U.S. Pat. No. 3,403,289, Distortion Correction System For Flying Spot Scanners, issued Sept. 24, 1968 to Gerald A. Garry. This method applies the uncorrected major deflection signals to two different correction circuits. The first correction circuit produces a correction for the major deflection signal and the second produces a signal which is input to the minor deflection signal generating circuits to produce a corrected minor deflection signal. This system requires specialized correction modules and minor deflection signal generators. In a third method, described in U.S. Pat. No. 3,487,164, Display Apparatus Deflection Signal Correction System With Signal Multiplication, issued Dec. 30, 1969, to C. A. Eggert, the corrected major deflection signals are analog multiplied with the uncorrected minor deflection signals and the results are summed to produce a corrected minor deflection signal. This method does not apply a separate correction function to the minor deflection signal. It is also possible to correct pincushion distortion by the placement of permanent magnets so as to form the proper fields to deflect the electron beam.